Display modules are used for instance in different computer systems and portable terminals, such as mobile stations, for displaying text and images to the user of the device. A display signal, typically comprising a display data signal and a synchronization signal, is input in a display screen comprised by the display module. A display data signal comprises image frames, which are distinguished from each other by means of a vertical synchronization signal comprised by the synchronization signal. The synchronization signal also comprises a horizontal synchronization signal for specifying the mutual separation of lines comprised by one image frame.
In conventional display systems, synchronization signals are typically incorporated into a display signal coming to the display module, whereas in newer display modules comprising a frame memory, synchronization signals are generated inside the display module. The frame memory is used for buffering image frames before the image frames are displayed on the display screen. Image frames are displayed to the user of the device by means of the display screen, the display screen receiving the image frames at a given rate. Image frames are updated to the display screen at a rate called the display refresh rate.
The problem here is that the update of a new image frame in the frame memory easily gets unsynchronized relative to the display refresh rate, which causes what is known as a tearing effect in the image displayed on the display screen, one part of the image displayed on the screen being composed of one image frame and another part being composed of another image frame. This is seen on the display screen as flickering lines or breaking in a laterally moving object. The tearing effect is generated because data comprised by a new image frame is stored in the frame memory before the corresponding data in the previous image frame is updated to the display screen.
The Applicant's previous patent application EP 1217602 discloses a method of avoiding tearing in a display unit. In the solution of publication EP 1217602, synchronization signals are connected to a control system from the display signal to be updated to the display screen, feedback synchronization signals being combined to form one signal, allowing the control system to interpret both synchronization signals correctly from one signal. The synchronization signals can be combined to form one signal by means of a logic OR or XOR operation, for example. The control system is timed on the basis of pulses comprised by the synchronization signals to input a new image frame in the frame buffer.
However, a drawback of this simple and thus advantageous solution is that the update of the following image frame in the buffer memory is controlled on the basis of a predetermined pulse, typically a vertical synchronization signal pulse. In this case, during the update of one image frame, there is only one moment that can trigger the update of a new image frame. In other words, if the application used presents an update request for a new image frame soon after the vertical synchronization signal pulse, the update of the image frame is initiated only from the following vertical synchronization signal pulse. Such a long delay in an image frame update causes harmful jerking in the image displayed on the display.